We have a theory of grand unification.
We don't. We have several candidate theories, of which the simplest ones are known
false (proton decay). Also even in situations where we do have good theories for the underlying physics getting from that to observable predictions can be quite painful. No one has been able to calculate the proton mass for examplpe.
Had inflation occured within what is describable that way, we'd already have a theory of inflation.We are if we ask, will the inflation be eternal or not?
We can't go from QCD -> mass of proton yet.
If you think that is not true, give me one experiment that has been done or could be done with current technology that definitiverly comes out A if inflation is eternal, and not A if it isn't.
The Guth paper points out that observation of curvature would rule out some models of eternal inflation.
What I said is unknowable is that is going on in domains that we cannot observe.
It's possible to make strong inferences about things that you can't directly observe. For example, we can't observe the core of the earth directly, but that doesn't prevent us from saying meaningful things about it.
We already can make statements about parts of the universe outside of the observation radius.
Which will again be a model, just like the current flat model is, and it will again not really tell us what is going on in the regions we cannot observe, just as the current model cannot.
If you keep insisting that the current cosmological model requires that the universe be flat, then this conversation is going to go nowhere. I'm about to give up here.
If we detect some miniscule curvature in the observable universe, why on Earth would we extrapolate that, as we would need to, to a volume hundreds or thousands of times larger than what we can observe?
Because we can tell from observational data how much the universe inflated, and then this gives you the radius at which you can extrapolate local observations.
Also, a lot of scientific statements are of the form, if X then Y. If you argue that curvature can't be extrapolated, then you *must* believe that the universe is non-isotropic. You can then look for signs of non-isotropy.
Calculate the precision in the curvature you would need at the end of inflation to produce a flatness that was within, say, 0.1% of 1 today.
It's on the order of 10^-18. That's not zero.
The mass of the electron is 10^-31 kg. That's not also zero.
Also, the amount of curvature that you would need to produce a flatness that is within a factor of 100 of 1 is 10^-14. If you have any flatness that is within a factor of a million of 1, you are going to have something to explain.
That is the whole reason for the invention of anthropic thinking, to be able to have a straight face as we say that the numbers are unexpected by 100 orders of magnitude.
You are missing the point. The reason inflation is cool is that it *avoids* the need for anthropic thinking. Without inflation, you have to have this weird coincidence to have any sort of curvature that is within a factor of a million of what we observe. With inflation, you don't.
You mean the papers that refer to eternal inflation? They just make my point-- they are based in anthropic thinking, which is required to get nonflat universes from inflation.
No they don't. If you want, you can just say that the universe works that way. Also since the inflation mechanism is unknown, the statement that anthropic thinking is required to get non-flat universes is something without any basis.
Sure you can get it published, but it is very far from mainstream astronomy, and I personally know few astronomers who would ever teach anthropic models of the universe to a class
I know the cosmologist that published the paper that made anthropic models respectable. He has a Nobel prize in physics, and I presume he teaches the topic in his class in cosmology.
At this point, I don't think it's possible to have a decent class in cosmology without mentioning the anthropic principle. I'm personally skeptical of anthropic arguments, but it's an idea with enough backing that you can't avoid teaching it.
I've told you why I reject that as mainstream science, as does almost every astronomer I know (I know a lot, outside the subfield of speculative cosmology).
I don't *like* the anthropic principle, but it's certainly not "outside the bounds of mainstream science."
do you still hold to your claim in the absence of anthropic thinking, i.e., in a model where you just get one universe, not 10100 to pick from to get the result you want?
Without inflation, you either have to chose fine-tuning or anthropic arguments to get any flatness < million. With inflation, you don't have to fine tune or use anthropic arguments to get that result. That's good.
Your statement would only have been true had we been able to observe the whole universe, which we already know we cannot do. You apparently think that if we observe a tiny curvature, it means the whole universe, beyond what we can observe, will match that same curvature.
No I don't. If we observe a tiny curvature, and the universe is isotropic and homogenity, then everything will match that curvature. We then look observational results which measure isotropy and homogenity to see what the limits on that are.
If it turns out that the universe is finite, then we could using observations to establish that the universe is isotropic within the radius of curvature of the universe.
Correction-- whether the observable universe is curved or flat is purely an observational issue! We already know what the whole universe is doing is not an observable issue, that's the point.
Not true. If the universe is finite then we can measure the entire universe. If it isn't then we can't. We don't know whether the universe is finite or not.
What's more, the current evidence is that it is flat, a point that you seemed to dispute earlier.
And I dispute it now. The current evidence is that the universe is within 0.01 of being flat. That's different from saying that it's flat. Also, there are some assumptions in the evidence that may not be true. The calculations assume GR is correct and that dark energy is the cosmological constant. If those are false, then the numbers could change.
As of 1995, the best numbers were that the universe had a curvature of -0.7. If it turns out that we aren't seeing dark energy, then we go back to those numbers.
Whether or not it could be curved, and inflation still be a good model, seems to be a matter of whether or not one views anthropic thinking as valid scientific reasoning.
You are changing your assertions. That's not necessarily a bad thing, but it will save me some effort if you now admit that non-zero curvature does not exclude inflation.
If you concede this point, then I don't see why raise anthropic principles. Guth only does so in his paper to reduce the search space of possible parameters.
Personally, I strongly dislike anthropic arguments. So lets reject the anthropic principle, and let's suppose we observe a positive curvature, I don't see the impact on inflation. There's enough evidence for inflation in the form of CMB background that I don't see what the issue is.